Brake feedback valve

ABSTRACT

A brake apply control system for a vehicle. The control system has a selectively operable mechanical brake apply portion and a selectively operable hydraulic brake apply portion. The mechanical brake apply portion includes an operator linkage for connecting the operator with a mechanical brake apply mechanism and a hydraulic piston and valve combination for providing a hydraulic feedback force to the operator. The hydraulic brake apply portion includes a source of hydraulic pressure, a brake apply valve for supplying a selective brake signal of variable intensity in response to operator input, a conduit system which connects the source of hydraulic pressure to the hydraulic piston and valve combination and which communicates the brake signal to the hydraulic piston and valve combination. The hydraulic piston and valve combination responds not only to the source of hydraulic pressure to provide a first feedback force to the operator, but also to the brake signal for providing a second feedback force to the operator that is proportional to the intensity of the brake signal.

TECHNICAL FIELD

The present invention relates generally to vehicular braking systems.More particularly, the present invention relates to a brake feedbacksystem that may be readily incorporated in hydraulically andmechanically actuated vehicular braking systems. Specifically, thepresent invention relates to a mechanism by which a hydraulic feedbackvalve and piston is applied to the brake apply linkage in a vehicularbraking system to maintain a feedback force to the operator when thehydraulic actuation begins.

BACKGROUND OF THE INVENTION

The present invention is particularly adapted for use in conjunctionwith torque transfer devices in the nature of multi-plate, brake packsthat are hydraulically and/or mechanically actuated. Such multi-pack,brake packs are often employed with the individual transmission outputshafts, or the axle assemblies connected thereto. Such brake packs aregenerally actuated by axial compression to effect the desired brakingaction in response to the depression of a brake pedal. That is, thecompression of the multiple plates with the friction disks therebetweeneffects the torque transfer which actually slows the vehicle.

The durability, and hence the life span, of such multi-plate, brakepacks is dependent upon the application of a coolant, generally acooling fluid, to the brake packs prior to the actuating compressionthereof. Simply stated, the heat that can be generated within brakepacks which are not adequately precooled will distress the successivelystacked plates and friction disks therein and ultimately lead to anuntimely failure of the brake pack.

The preferred brake apply valves do make provision for pre-cooling, butsuch pre-cooling is induced by the initial depression of the brakepedal. That type arrangement can work quite well under appropriateconditions. However, in those instances where the driver depresses thebrake pedal much more rapidly than normal--as in a panic stop--thepre-cooling flow might not be initiated in time for the necessarycoolant to have reached the brake packs prior to their compression.

Other conditions which might also delay the desired flow of the coolantuntil after compression of the brake packs has been initiated are:positioning the brake coolant valve at a location which is sufficientlyremote from at least one brake pack that the pre-cooling bath can notbegin prior to compression of the brake packs; or, using a coolantdelivery system configuration which allows the cooling fluid to drainfrom a significant portion of the system when the brakes are not beingapplied, thus requiring that the system replenish the fluid thereinbefore the pre-cooling bath can begin.

Also, in many track laying vehicles, brake systems employ bothmechanical and hydraulic apply mechanisms. With these integratedsystems, the usual applied sequence is mechanical before hydraulic. Themechanical applied portion will generally use a cam-apply mechanismwhich will increase mechanical advantage. When the hydraulic appliedportion comes into effect, the mechanical input reaction is reduced suchthat the operator must be alert to the change in reaction force which isfelt at the brake pedal. While experienced operators are not overwhelmedby this system, it would be preferable to alleviate this condition.

SUMMARY OF THE INVENTION

The present invention seeks to overcome this reduction in reaction forceby supplying a hydraulic feedback force substantially equal to the lostmechanical feedback force. This is accomplished by incorporating ahydraulic valve and piston assembly into the mechanical linkage. Thehydraulic valve will emit fluid pressure to the piston substantiallysimultaneously with the commencement of the hydraulic applied portion.Thus, the application of hydraulic force to the brake system iscompensated for by a hydraulic feedback signal to the operator thuspreventing the loss of the feedback force or reaction at the brakepedal.

It is therefore an object of this invention to provide an improvedfeedback force mechanism in a mechanical/hydraulic brake operatingsystem.

It is another object of this invention to provide an improved brakeapply system wherein the hydraulic valve and piston assembly isincorporated into a mechanical apply linkage such that a hydraulicsignal proportional to the mechanical reaction force within the brake isapplied to the mechanical linkage.

It is a further object of this invention to provide an improvedhydraulic/mechanical braking system wherein a hydraulic feedback signalis incorporated into the mechanical linkage to compensate for thereduction in mechanical reaction forces within the braking system.

These and other objects of the invention, as well as the advantagesthereof, over existing prior art forms will be apparent in view of thedetailed specification and accompanying means hereinafter described andclaimed.

The present invention is described in conjunction with one exemplaryembodiment of a brake control system which is deemed sufficient toeffect a full disclosure of the subject invention. The exemplary brakecontrol system is described in detail without attempting to show all ofthe various forms and modifications in which the invention might beembodied; the invention being measured by the appended claims and not bythe details of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing representing the major components utilizedin a throttle responsive, as well as a brake apply, pre-cooling systemfor a hydraulically and/or mechanically actuated vehicular brakingmechanism, the arrangement depicting the system under the condition thatno operating pressure is being applied to the brake pedal, and thethrottle pedal has been substantially depressed--as would be thesituation when the vehicle is moving under power;

FIG. 2 is an enlarged portion of FIG. 1 depicting the brake apply valvein schematic, axial cross section;

FIG. 3 is a view similar to FIG. 2, but depicting the components of thebrake apply valve disposed in response to the initial depression of thebrake pedal by the vehicle operator;

FIG. 4 is a view similar to FIGS. 2 and 3, but depicting the componentsof the brake apply valve disposed in response to continued depression ofthe brake pedal;

FIG. 5 is an enlarged portion of FIG. 1 depicting the components of thebrake coolant valve disposed in response to continued depression of thethrottle pedal and without depression of the brake pedal;

FIG. 6 is a view similar to FIG. 5, but depicting the components of thebrake coolant valve disposed as they are in response to depression ofthe brake pedal and/or in response to the removal of actuating pressurefrom the throttle pedal;

FIG. 7 is a schematic cross section taken axially through that portionof a transmission case which houses a brake assembly that acts upon thetransmission output shaft in proximity to its connection with an axleassembly of a vehicle, the cross section depicting a representativebrake apply assembly employing opposed camming ramps to effect themechanical actuation of the brake pack and an actuating cylinder toeffect hydraulic actuation of the brake pack, the mechanical andhydraulic actuating systems being compatibly cooperative;

FIG. 8 is an enlarged area of that portion of the schematic crosssection of FIG. 7 defined generally by the circle designated as "FIG-8"therein;

FIG. 9 is an enlarged, partially exploded perspective of the brake applyassembly depicted in FIGS. 7 and 8 removed from the housing, and withselected components of the brake apply assembly disposed in theirjuxtaposed, operative relationship;

FIG. 10A is an exploded perspective of that portion of FIG. 9 designatedas "FIG-10A" and depicting a portion of the brake apply assembly removedfrom the housing;

FIG. 10B is an exploded perspective of that portion of FIG. 9 designatedas "FIG-10B" and depicting another portion of the brake apply assemblyremoved from the housing;

FIG. 11 is a schematic, elevational representation of a portion of thebrake assembly to assist in explaining the self-energizing features ofthe brake assembly as well as the mathematical expressions whichdelineate that self-energization;

FIG. 12 is an enlarged portion of FIG. 1 depicting a modulator signalvalve and the combined cut-off/timer valve assembly in schematic, axialcross section;

FIG. 13 is a view similar to FIG. 1, but with the modulator signal valvehaving been translated by control pressure within the actuating chamberto admit fluid under mainline pressure into the directing chamber;

FIG. 14 is a view similar to FIGS. 1 and 13, but depicting actuation ofthe timer valve portion sufficient to close the cut-off valve portion;

FIG. 15 is a view similar to FIGS. 1, 13 and 14, but depicting themodulator valve and the combined cut-off/timer valve assembly reset, andwith the brake apply valve actuated to supply coolant to the brakepacks; and,

FIG. 16 is a curve depicting the relationship between pedal travel andpedal force.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT

One representative form of a brake cooling system embodying the conceptsof the present invention is designated generally by the numeral 10 onthe accompanying drawings. The representative brake cooling system 10,schematically depicted in FIG. 1, may employ a brake apply valve 100that sequentially stages the flow of coolant to the brake packs 11(hereinafter shown and described in conjunction with FIGS. 7, 8 and 11)upon initial depression of the brake pedal 12. The brake apply valve 100also controls the flow of fluid which effects hydraulic actuation of thevehicular brake packs 11 in response to continued depression of thebrake pedal 12. The structural description, as well as the aforesaidfunctions, of the brake apply valve 100 will be hereinafter described indetail.

The brake cooling system 10 also employs a brake coolant valve 200. Thebrake coolant valve 200 directs, the actual flow of the coolant fluid tothe brake packs 11. The brake coolant valve 200 and the means by whichit is actuated will also be hereinafter more fully explained.

The present brake cooling system 10 uniquely responds to the controlpressure provided by a computerized control member 13 in response to theactuation pressure applied to the throttle pedal 14. Specifically, thethrottle responsive control pressure is fed from the control member 13to a modulator signal valve 400. The modulator signal valve 400 operatesin conjunction with a combined cut-off/timer valve assembly 500 tosupply a pre-cooling bath to the brake packs 11 in response to theoperator's operation of the throttle pedal 14. The structure, and theoperation, of the modulator signal valve 500 and the cutoff/timer valveassembly 400, will also be hereinafter described in detail.

It should be noted that the hydraulic fluid employed to actuate thebrakes may also be the source of the fluid employed to cool the brakes,and that fluid may be stored in a reservoir 15, as best seen in FIGS.2-4. A pump 16 is generally employed to supply hydraulic fluid from thereservoir 15 to the brake apply valve 100 at the desired mainlinepressure. The means by which the present brake cooling system 10supplies a pre-cooling bath to the brake packs 11 will become apparentby virtue of the detailed description which follows.

Several peripheral components may be effectively employed in the overallbrake cooling system 10 which will, for overall clarity, be describedindividually. It has been deemed appropriate to describe the operationof each such component in conjunction with the structural descriptionthereof. There will, of course, be a brief description as to theoperation of the overall invention at the end of the specification, butin order to preclude the necessity for an overly cumbersome descriptionat that point, the election was made to provide an operationaldescription for each component, as that component is described.

Brake Apply Valve

Turning now to an explanation of the brake apply valve 100, it must beunderstood that the brake apply valve 100 controls the application ofpressurized hydraulic fluid, such as oil, to the cylinders 101 whichoperate the brake packs 11 of a vehicle. The brake apply valve 100 alsoprovides a brake apply signal pressure to operate a brake coolant valve200 which, in turn, controls the application of a coolant--normallycooled hydraulic fluid available from the same source as the hydraulicfluid flowing to, or through, the brake apply valve 100--to theengageable torque transfer device employed by the brake packs 11 of thevehicle.

As depicted in FIGS. 2 through 4, the brake apply valve 100 employs pairof axially aligned, and spaced, first and second spool members 102 and103 that are received within a housing 104 for axial translation. Thefirst spool member 102 has a pair of axially spaced, first and secondcylindrical lands 105 and 106, respectively, of equal diameter whichslidingly engage a first, cylindrical, interior wall portion 108 of thehousing 104 to define a brake apply signal chamber 110 between the lands105 and 106.

The first spool member 102 has a head portion 111, the outwardlydirected face 112 of which may be engaged by a low friction rollerpresented from an actuating arm 113. The roller 109 applies a displacingforce to the spool member 102 in response to the application of force tothe brake pedal 12 by the operator. When the actuating arm 113 ismechanical, as depicted in the drawings, it may be mounted for rockingaction in response to depression of a brake pedal 12 by the operator ofthe vehicle. The connection between the brake pedal 12 and the actuatingarm 113 may be accomplished by link means, such as shown at 114. Theactuating arm 113 may, as shown, be mounted on an extension of the applyshaft 323, that is more fully shown and described in conjunction withthe mechanical and hydraulic apply system 300 depicted in FIG. 10A, orthe actuating arm 113 may be connected by other suitable force transfersystems to the apply lever assembly 324 that is also shown and describedin conjunction with FIG. 10A.

The head portion 111 of the first spool member 102 may be annularlyrecessed, as at 115, to present an annular rim 116 that is engaged by acompression, return spring 118 which acts between the opposed, annularrim 116 and a shelf 119 presented from the housing 104. The returnspring 118 applies a biasing resistance to translation of the firstspool member 102, and that resistance may be reflected againstdepression of the brake pedal 12 in order to provide a tactile feed-backto the operator. Moreover, the return spring 118 tends to maintain thefirst spool member 102 in, or to return the first spool member 102 to,the unactuated state depicted in FIG. 2.

The second spool member 103 has a pair of axially spaced, first andsecond cylindrical lands 120 and 121 of unequal diameter. The lands 120and 121 are slidably disposed in sealing engagement with a steppeddiameter bore in the housing 104. Specifically, the lands 120 and 121cooperate with the interior of the housing 104 to define a brake applychamber 122 therebetween. The first land 120, which is of greaterdiameter than the second land 121, slidingly engages a second,cylindrical, interior wall portion 123 presented from the housing 104,and the second land 121 slidingly engages a third, cylindrical, interiorwall portion 124 presented from the housing 104. The functional purposeof the differential areas presented to the brake apply chamber 122 bythe first and second lands 120 and 121, respectively, will behereinafter more fully described.

A connecting rod 125 is anchored in the first spool member 102, as bythe threaded attachment 126, and the connecting rod 125 extendsslidingly through an axial bore 128 in the second spool member 103 toterminate in a retaining cap 129 that may also be secured to theconnecting rod 125, as by the threaded attachment 130. The retaining cap129 engages one end face 131 on the second spool member 103 to delineatethe limit to which the second spool member 103 can separate axially fromthe first spool member 102.

A regulating compression spring 132 is interposed between the first andsecond spool members 102 and 103. As depicted, the interior of the firstspool member 102 may be axially recessed, as at 133, such that thesecond land 106 is supported from a skirt portion 134. The regulatingspring 132 is received within the axial recess 133 and extends axiallyoutwardly therefrom to engage a centering pedestal 135 that extendsaxially outwardly from the other end face 136 of the second spool member103. The functional operation achieved by having the regulating spring132 continuously bias the two spool members 102 and 103 apart will alsobe hereinafter more fully described.

The unactuated disposition of the components in the brake apply valve100 is determined by the unopposed biasing action of the return andregulating springs 118 and 132, respectively, as represented in FIG. 2.In the unactuated state of the brake apply valve 100 the brake signalchamber 110 communicates with the brake coolant valve 200 by virtue of abrake apply signal feed conduit 140, and the brake signal chamber 110also communicates with the hydraulic return system 141 through anexhaust conduit 142. Specifically, the exhaust conduit 142 opens to thebrake signal chamber 110 through an exhaust port 143. In the unactuatedstate of the brake apply valve 100, therefore, the brake apply signalchamber 110 provides a path by which the hydraulic fluid which actuatesthe brake coolant valve 200 can enter the hydraulic return system 141 todeactivate the brake coolant valve 200 and thereby terminate the flow ofcooling fluid to the brake packs 11. The specific, structural details ofthe brake coolant valve 200 are also hereinafter more fully described.

In the unactuated state of the brake apply valve 100, the brake applychamber 122 communicates with the brake cylinders 101A and 101B byvirtue of a brake apply feed conduit 144. The feed conduit 144 opens tothe brake apply chamber 122 through an outlet port 145. The brake applychamber 122 also communicates with the hydraulic return system 141, butthrough a second exhaust conduit 146 that opens to the brake applychamber 122 through an exhaust port 148. The exhaust conduit 146includes a check valve 149 which allows the actuating pressure to berelieved from the brake cylinders 101 but which precludes the brakecylinders 101 from emptying. This allows for virtually instantaneousresponse by the brake cylinders 101 upon the application of actuatingpressure through the brake apply valve 100, as will be hereinafter morefully described.

In the detailed description which follows, a particular structuralmember, component or arrangement may be employed at more than onelocation. When referring generally to that type of structural member,component or arrangement a common numerical designation shall beemployed. However, when one of the structural members, components orarrangements so identified is to be individually identified it shall bereferenced by virtue of a letter suffix employed in combination with thenumerical designation utilized for general identification of thatstructural member, component or arrangement. Thus, there are at leasttwo brake cylinders which are generally identified by the numeral 101,but the specific, individual brake cylinders are, therefore, identifiedby the alphanumeric designations 101A and 101B in the specification andon the drawings. When two quite similar, or even identical, componentsare closely related to a third component, the two similar componentsshall be identified by the same numerical designation as the componentto which they are related, except that the similar components shall bereferenced by virtue of a letter subscript employed in combination withthe numerical designation utilized for identification of the relatedcomponent. These same suffix conventions shall be employed throughoutthe specification.

As previously noted, the source of hydraulic fluid employed to actuatethe brakes may also be the source of the fluid employed to cool thebrakes, and that fluid may be stored in a reservoir 15 that is fed bythe hydraulic return system 141. A pump 16 is generally employed tosupply hydraulic fluid from the reservoir 15 to the brake apply valve100 at the desired mainline pressure. As shown, the pressurizedhydraulic fluid from the pump 16 is fed into the brake apply valve 100by branches 150_(A) and 150_(B) of a supply conduit 150. In theunactuated state of the brake apply valve 100, pressurized hydraulicfluid is not provided access to either the brake signal chamber 110 orthe brake apply chamber 122. Instead, the second land 106 on the firstspool member 102 blocks the inlet port 151 by which the first branchconduit 150_(A) opens through the first, cylindrical, interior wallportion 108 of the housing 104 selectively to feed the brake signalchamber 110. Similarly, the second land 121 on the second spool member103 blocks the inlet port 152 by which the second branch port 150_(B)opens through the third, cylindrical, interior wall portion 124 of thehousing 104 selectively to communicate with the brake apply chamber 122.

With particular reference to FIGS. 1 through 3 it can be seen that ahydraulic valve and piston assembly 160 is employed in the brakingsystem. This hydraulic valve and piston assembly 160 includes a housingportion 161 in which is formed the stepped diameter bore 162 including asmall diameter bore 163 and a large diameter bore 164. An operator inputpiston, or plug member 166, is slidably disposed in the small bore 163.A plunger sleeve valve 168 is slidably disposed within the large bore164. The sleeve valve 168 includes a central bore 190 in which isslidably disposed a main pressure pin 191 and a balance, or reaction,pin 193. The balance pin 193 has a snap ring 194 secured thereto whichlimits rightward (as viewed in the drawings) movement of the pin 193relative to the sleeve valve 168. The end of bore 164 is closed by aplug 195 which is secured in the housing 161 by a snap ring or otherconventional ring-locking device 167. The plug 195 limits the leftward(also as depicted in the drawings) movement of the pin 193.

The sleeve valve 168 is urged rightward by a spring 170 so as to causethe right end 171 of the valve 168 to abut a shoulder 173 formed betweenthe small bore 163 and the large bore 164. The sleeve valve 168 has aradial passage 174 which will direct pressure from an undercut 175,formed in the outer surface of the valve 168 to the bore 190. In thespring set position shown in FIG. 1, the undercut 175, and thereforepassage 174, is connected with an exhaust passage 177. The spring 170 isdisposed in a chamber 178 which is also connected with an exhaustpassage 180. This will permit the exhausting of any fluid leakage pastthe sleeve valve 168 into the chamber 178, thereby preventing a buildupof fluid pressure in this area.

The valve bore 164 is also disposed in fluid communication with apressure passage 181 which is connected directly with the main linepressure conduit 150. The outer diameter of a surface of sleeve valve168 blocks the passage 181 from permitting fluid to enter the undercut175 when the valve is disposed in the position shown in FIG. 1. Theshoulder portion 173 of the valve body 161 is provided with a fluidconnection to a passage 183 which is connected to the brake apply feedcircuit 144. Thus, whenever the brake apply feed is pressurized, fluidpressure will enter the passage 183 to operate on the right end 171 ofthe sleeve valve 168. The pressure that acts on the right end of thevalve sleeve 168 will also act on the left-most face 184 of the piston166.

The piston 166 is connected through a link 185 to an arm 186 which inturn is secured for rotation with the actuating link 113. Thus, wheneverthe brake pedal 112 is depressed, the arm 186 and link 185 will enforcemovement of the piston 166 into the bore 163.

Also with actuation of the brake pedal 12 fluid pressure is admitted bythe brake apply valve 100 to the passage 144 and therefore passage 183.The fluid pressure within passage 183 will be operable to move thesleeve valve 168 leftward against the spring 170 to the position shownin FIG. 4. In this position the main line pressure available in passage181 is communicated with the undercut 175 and therefore the radialpassage 174. As seen in FIG. 4, the passage 174 aligned with the spacebetween main pressure pin 191 and reaction pin 193. The main pressureacting in this area will cause the main pressure pin 191 to be urgedrightward against the piston 166. This urging of the pin 191 will causethe feedback force to be asserted or otherwise applied to the link 185and arm 186.

As previously discussed, the arm 186 is rotatable with the link 114which is connected to the pedal 12. Thus, the force applied to the pin191 by the main pressure end passage 181 causes a feedback force to beapplied to the operator pedal 12. Likewise, when the brake apply valve100 is opened and the pressure and passage 183 is applied to the sleevevalve 168, the pressure is likewise applied to a portion of the brakeapply feedback piston 166. This force is also felt as feedback force bythe operator. Thus, the hydraulic valve and piston assembly 166 suppliestwo feedback forces to the operator. One of the forces is asubstantially constant force and is proportional to main line pressureand the other is a variable force proportional to the brake applypressure which is present in passage 183.

As seen in FIG. 16, during initial or mechanical engagement of thevehicle brake the pedal force to pedal travel relationship isessentially a straight-line relationship 187 and is proportional to thecompression springs 382 which serve as return springs within the brakeassembly. However, when the brake apply valve 100 is opened and ahydraulic apply force is directed to the brake assembly, the reactionforce in the reaction cam member 343 is substantially reduced such thatunder normal conditions the only feedback force felt by the operatorwould be a result of the hydraulic forces within the brake apply valve100. With continued reference to FIG. 16, however, it can be observedthat a pressure proportional to the system main pressure is applied tothe pin 191 such that the dropoff, or decrease, in reaction force is notfelt by the operator.

It is also seen in FIG. 16 that the brake apply pressure which isdirected to the piston 166 provides a substantially straight-linerelationship between the pedal travel and pedal force along the curve189. Thus, as is represented by the curves shown in FIG. 16, and thestructure of the feedback valve and piston 160 described above withregard to FIGS. 1 through 4, the operator does not have a physicalindication that the hydraulic brake apply has relieved the reactionforce from the reaction cam 343. As seen in FIG. 16, the pedal travelversus pedal force relationship remains as a substantially constantstraight line without any significant changes and therefore the feedbackfelt by the operator is a substantially straight-line function.

When the vehicle operator initially depresses the brake pedal 12 theroller 109 on the actuating arm 113 applies a force against theoutwardly directed face 112 on the head portion 111 of the first spoolmember 102. The force applied by the actuating arm 113 translates thefirst spool member 102, as depicted in FIG. 3, when that force issufficient to overcome the biasing action of the return spring 118. Asthe first spool member 102 is thus translated, the first land 105thereon blocks the exhaust port 143, thereby closing communicationbetween the brake signal chamber 110 and the hydraulic return system141. That same translation of the first spool member 102 also translatesthe second land 106 away from the inlet port 151 to permit communicationbetween the first branch 150_(A) and the brake signal chamber 110.Mainline hydraulic pressure is thereupon transmitted through the brakesignal chamber 110 and the brake apply signal feed conduit 140 to openthe brake coolant valve 200 and allow cooled hydraulic fluid to bathe,and cool, the brake packs 11. This operation of the brake coolant valve200 is also hereinafter described.

Any hydraulic fluid which may inadvertently accumulate within the recess133, or the space 153 between the first and second spool members 102 and103, continuously empties into the hydraulic return system 141 throughthe third exhaust conduit 154. As the first spool member 102 translatesin response to depression of the brake pedal 12, the connecting rod 125will slide along the axial bore 128 which extends through the secondspool member 103, and only the biasing action of the regulating spring132 will effect translation of the second spool member 103 toward thethen displaced retaining cap 129 that determines the extent to which thesecond spool member 103 can move axially away from the first spoolmember 102.

As shown in FIG. 4, the regulating spring 132 initially translates thesecond spool member 103 such that the first land 120 thereon closes theexhaust port 148 by which the exhaust conduit 146 opens through thesecond cylindrical interior wall portion 123, thereby closing the brakeapply chamber 122 to the hydraulic return system 141. As is alsorepresented in FIG. 4, continued translation of the second spool member103 translates the second land 121 thereon to open the inlet port 152 topermit the introduction of pressurized hydraulic fluid from the secondsupply branch 150_(B) into the brake apply chamber 122. The foregoingdescription delineates an arrangement wherein an "underlap" exists as tothe spacing of the lands 120 and 121 relative to the spacing of therespective ports 148 and 152 with which the lands 120 and 121 interact.

It is also possible to space the lands 120 and 121 relative to the ports148 and 152 such that they are "line-on-line." That is, the lands 120and 121, and/or the ports 148 and 152, may be spaced such that at theinstant one port closes, the other port is opening. Finally, it ispossible to effect a disposition which constitutes an "overlap." In anoverlap disposition the land 121 would open port 152 just prior to theclosure of port 148 by land 120.

These three relationships of the lands to the ports are well known tothe art, and they are mentioned herein merely to establish that thebrake apply valve will operatively accommodate any of the threerelationships to accomplish any of the objectives achieved by thosethree relationships.

Because the check valve 149 does not permit either the brake cylinders101 or the brake apply feed conduit 144 to empty, the pressurized fluidintroduced into the brake apply chamber 122 is applied virtuallyinstantaneously to the brake cylinders 101 through the brake apply feedconduit 144. As the downstream pressure within the feed conduit 144increases, that pressure will be reflected in the brake apply chamber122 to be applied against the projected areas of the lands 120 and 121which define the opposed, axial boundaries of the brake apply chamber122.

Because the projected area of land 120 exposed to the brake applychamber 122 is greater than the projected area of land 121 exposed tothe brake apply chamber 122, the hydraulic pressure within the brakeapply chamber 122 acts on that differential area to create a force thatmoves the second spool member 103 against the biasing action of theregulating spring 132. The displacement of the second spool member 103toward the first spool member 102 will depend upon the relative biasingpressure of the regulating spring 132 in comparison to the differentialforce applied to the second spool member 103 by the pressure of thehydraulic fluid with the brake apply chamber 122.

As long as the differential force exceeds the biasing action of theregulating spring 132, the second spool member 103 will be urged towardthe first spool member 102, even to the point of opening the exhaustport 148 which allows the fluid within the brake apply chamber 122 toexit into the hydraulic return system 141. However, as the pressurewithin the brake apply chamber 122 falls, the differential force actingon the opposed lands 120 and 121 of the second spool member 103 will beovercome by the biasing action of the regulating spring 132 to close theexhaust port 148 and reopen the second inlet port 152.

It must be appreciated that the translated location of the first spoolmember 102 directly controls the force which need be applied to theregulating spring 132 by the second spool member 103 in order to effectcommunication between the brake apply chamber 122 and either thehydraulic return system 141 or the supply branch 150_(B). Hence, thegreater the pressure applied by the operator to effect translation ofthe first spool member 102, the greater will be the brake apply pressurerequired in the feed conduit 144 to open the exhaust port 148 by whichto effect communication with the hydraulic return system 141.

As a result, the apply pressure directed to the brake cylinders 101through the brake apply valve 100 is regulated in response to the amountof force applied by the vehicle operator to the foot pedal 12. Inaddition, the operator is continuously supplied with tactile feed-backthrough the contact of his foot with the pedal 12. Such tactilefeed-back has been found to enhance the operator's visual observation ofthe vehicular speed reduction in response to his application of footpressure upon the pedal 12.

Brake Coolant Valve

To reiterate, the brake coolant valve 200 controls the flow of acoolant--normally the cooled hydraulic fluid available from the samesource as the hydraulic fluid flowing to, or through, the brake applyvalve 100--to the engageable torque transfer device--i.e.: the brakepacks 11 employed in the braking system of a vehicle--during theirapplication. The brake coolant valve 200 is actuated by the brake applysignal pressure emanating from the brake apply signal chamber 110 in thebrake apply valve 100, as previously described.

The mechanism of the brake coolant valve 200--as depicted in FIGS. 5 and6--may be contained within a multi-piece housing 201 that may beincorporated integrally with, or separate from, the housing 104 withinwhich the brake apply valve 100 is received. In either situation, thehousing 201 contains a first, or coolant delivery, chamber 202 and asecond, or lubricant delivery, chamber 203 separated by a transverse,medial wall 204 that is penetrated by a passage 205 through whichcommunication between the first and second chambers 202 and 203 can beselectively effected. A piston chamber 210 extends axially outwardlyfrom the coolant delivery chamber 202.

A shoulder 211 is presented in axially spaced relation from the medialwall 204 with the coolant delivery chamber 203 disposed between themedial wall 204 and the shoulder 211. A lubricant deliver sub-chamber212 extends axially outwardly from the lubricant delivery chamber 203past the shoulder 211. A first valve seat 213 is presented from themedial wall 204 in spaced opposition to a second valve seat 214presented from the shoulder 211.

A valve element 215 is translatable between the first and second valveseats 213 and 214 along the axis of a pilot pin 216 that is fixedlysecured to the housing 201. A compression spring 218 acts between thehousing 201 and the valve element 215 to bias the valve element 215 intooperative engagement with the first valve seat 213 which circumscribesthe passage 205 that communicates between the first and second valvechambers 202 and 203, respectively, within the housing 201 of the brakecoolant valve 200. As shown, the compression spring 218 may circumscribethe pilot pin 216 with one end received within a cylindrical anchoringrecess 219 in the housing 201 and with the other end received over acentering boss 220 provided on the underside of the valve element 215.The compression spring 218 continuously biases the valve element 215toward engagement with the first seat 213 in order to preclude flowbetween the second and the first valve chambers 203 and 202 through thepassage 205.

A valve operating piston 221 is received within the piston chamber 210that extends axially outwardly from the first, or coolant delivery,chamber 202 in the housing 201. The piston chamber 210 communicates witha feed conduit 140 that originates within the brake signal chamber 110of the brake apply valve 100. The valve element 215 is operativelyconnected to the piston 221, as by a surface engagement therebetween,such that the valve element 215 translates in direct response totranslation of the piston 221. As such, it may prove desirable for thepiston 221 to be integral with the valve element 215.

The first chamber 202 communicates with coolant feed lines 222 and 223that delivers the cooling fluid to the torque transfer device utilizedby the hereinafter described brake packs 11. A supply conduit 224 fromthe cooler 225 communicates with an entry chamber 226 to admit thecooled hydraulic fluid into the second chamber 203. When the brakecoolant valve 200 is closed, as depicted in FIG. 5, the main volume ofthe cooled hydraulic fluid entering the second chamber 203 through thehydraulic fluid supply conduit 224 from the cooler 225 flows into thelubricant delivery sub-chamber 212 and is then discharged through thelubricant distribution conduit 228 which communicates with thesub-chamber 212. In addition, a small portion of the cooled hydraulicfluid entering the second chamber 203 is delivered to the first chamber202 through a first, restricted orifice 229 in order to provide anuninterrupted supply of cooling fluid with which to bathe the brakepacks 11, even when it is not being applied.

When the brake apply valve 100 is operated in response to initialdepression of the throttle pedal 12, a brake apply signal pressure isprovided to the piston chamber 210 in the brake coolant valve 200 fromthe brake signal chamber 110 in the brake apply valve 100, as previouslydescribed. The brake apply signal pressure acts within the pistonchamber 210 to translate the operating piston 221 and displace the valveelement 215 away from the first valve seat 213 and into sealingengagement with the second valve seat 214, as shown in FIG. 6. This fulldisplacement of the valve element 215 fully opens the passage 205 andthereby permits the cooled hydraulic fluid in the second chamber 203 ofthe brake coolant valve 200 to flow into the first chamber 202 and outthrough the feed lines 222 and 223 to the brake packs 11. The resultingunrestricted flow of the cooled hydraulic fluid from the second chamber203 to the first chamber 202 allows virtually the full flow of thecooled hydraulic fluid through the supply conduit 224 from the cooler225 to be made available to cool the brake pads when they are beingapplied.

A second, restricted orifice 230 communicates between the entry chamber226 and the lubricant delivery chamber 212 to assure that at least asmall portion of the cooled hydraulic fluid will be provided for generallubrication, even when the brakes are applied. During application of thebrakes, therefore, a continued small portion of the hydraulic fluid ispermitted to pass from the entry chamber 226, through the secondrestricted orifice 230, into the lubrication delivery sub-chamber 212,and from there into conduit 228. This arrangement assures the continuedflow of at least a minimal quantity of lubricating fluid to theremainder of the system, even during the application of maximum brakingeffort.

In order to ensure that the valve element 215 will be properly displacedin response to the application of the signal pressure within the pistonchamber 210 it may be astute to provide a relief passage 231 which willallow any fluid that might inadvertently collect within the pilot bore232 which receives the pilot pin 216 to exit outwardly through therelief passage 231 and not obstruct the operation of the piston 221 orthe valve element 215. By selecting a suitable cross sectional area forthe relief passage 231 that passage can admit fluid into the pilot bore233 when the valve is in its unactuated state (FIG. 5), and modestlyrestrict the exiting flow of fluid from the pilot bore 232, therebyhydraulically dampening the translational opening movement of the valveelement 215. By thus damping the translation of the valve element 215 itwill not open the passage 205 too quickly nor will the valve element 215translate in response to any transient spike in the signal pressureapplied to chamber 210.

Brake Apply Ramp

A combined, mechanical and hydraulic brake apply assembly 300--which maybe incorporated in the brake cooling system 10--is depicted in FIGS. 7through 11. The brake apply assembly 300 effects the application ofbrake apply force to the brake packs 11 in response to both mechanicallyand hydraulically generated forces. The mechanically and hydraulicallygenerated forces may be individually applied, simultaneously applied orapplied in selected sequential and/or simultaneous combinations. Thebrake packs 11 are operatively associated with the output shafts 301 ofa cross-drive transmission, or vehicle. The details of the transmission,being well known to the art, are not depicted in the drawings attachedhereto. The brake assembly 300 is received within a brake housing 302that is typically located linearly adjacent the transmission casing 303so that rotation of the transmission output shaft 301 can be transmittedto the brake apply assembly 300 received within the brake housingassembly 302. One may, if desired, combine the brake housing 302 withthe transmission casing 303, but for simplification of the presentexplanation they will be deemed to be contiguous, but separate.

The shaft 301 is connected by a spline 307 to a carrier assembly 317 ofa planetary gear set 327. The planet carrier assembly 317 has piniongears 347 which mesh with a ring gear 357 and a sun gear. The sun gearis identified by the spline-teeth 306 formed on a sun gear shaft 304.The sun gear shaft 304 provides an input member for the planetary gearset 327. A sleeve shaft 329 is drivingly connected to the sun gear shaft304 by teeth 305 and is connectible through teeth 337 to a conventionaldrive shaft from a transmission, not shown. The sleeve shaft 329 ismovable axially to permit ease of connection between the planetary gearset 327 and the transmission in a well known manner. The spline-teeth306 on the sun gear shaft 304 also operatively engage the splines 308 ona hub member 309. A radially outer rim 310 of the hub member 309 isprovided with a plurality of axially extending splines 311 operativelyto engage the brake pack 11. Actuation of the brake pack 11, ashereinafter explained, will provide a direct connection between thecasing 303 and the sun gear shaft 304. This will create a braking effecton the carrier assembly 317, and therefore shaft 301, which will provideslowing of the vehicle.

The brake pack 11 is operatively connected between the hub member 309and a brake apply annulus 312. Specifically, the radially inner surfaceon the skin portion 313 of the brake apply annulus 312 may incorporatesplines, in the nature of axial slots, as at 314, to receive the matingsplines, in the nature of tangs, 315 of the first, annular torque plates316 and thereby assure that the first torque plates 316 are notrelatively rotatable with respect to the brake apply annulus 312. Toassure that the interaction between the brake apply annulus 312 and thesplines 315 is sufficient to withstand the loading to which the splines315 may be subjected, a plurality of axially oriented splines, or slots,314 are employed at circumferentially spaced intervals about theradially inner surface on the skirt portion 313 of the brake applyannulus 312, and the first torque plates 316 are provided with asufficient number of tang splines 315 to interact with those slotsplines 314, as is well known to the art. For simplification of theexploded perspectives only two torque plates 316 are depicted in FIGS. 9and 10, but as should be appreciated, a plurality of such plates 316 maybe stacked in operative relation with the hereinafter described secondtorque plates 318, as is also well known to the art. As depicted in FIG.7, seven, first--or reaction--torque plates 316 are interleaved witheight, second--or drive--torque plates 318.

The splines 311 on the radially outer rim 310 of the hub member 309 arealso spaced at circumferential intervals to receive the several splines,in the nature of tangs, 319 which extend radially inwardly from each ofthe plurality of second, annular torque plates 318 and thereby assurethat the second torque plates 318 will not rotate relative to the hubmember 309, and thus the transmission output shaft 301, as is also wellknown to the art.

Annular friction disks 320 are preferably interposed between each of thesuccessive first and second torque plates 316 and 318, respectively. Toassure that the several friction disks 320 will not be displaced whenthey are not compressed between the successive first and second torqueplates 316 and 318, a friction disk 320 is, as a general rule, securedto both sides of the alternate first or second torque plates 316 or 318.By thus securing the friction disks 320 to only one of the torque plates316 or 318 the chance of having any friction disk 320 directly engageanother friction disk 320 is obviated. It is, of course, also possibleto apply one friction disk 320 to only one side of each torque plate 316and 318. In this arrangement care must be exercised to assure that onlyone friction disk is sandwiched between successive torque plates 316 and318 in order to preclude direct engagement between friction disks 320.

An annular backing plate 321 (FIG. 7) is also preferably supported bythe brake housing assembly 302 to provide a fixed member against whichthe brake pack 11 may be compressed. As shown, one of the friction disks320 may also be secured to the backing plate 321. The specificinteraction and operation of the structural members in the present brakeapply assembly 300 by which that compression is effected will behereinafter more fully explained.

As previously explained, a brake apply valve 100 provides a brake applysignal pressure to operate a brake coolant valve 200 which, aspreviously described, controls the application of a coolant--normallycooled hydraulic fluid available from the same source as the hydraulicfluid flowing to, or through, the brake apply valve 100--to the torquetransfer devices employed in each brake pack 11. In the embodimentdepicted, the torque plates 316 and 318 as well as the friction disks320 constitute a brake pack 11. A plurality of ports 322 extend radiallythrough the rim 310 of the hub member 309 to dispense the cooling fluidradially outwardly over the brake pack 11.

Focusing more specifically on the brake apply assembly 300, whichincludes the brake apply annulus 312, the apply shaft 323 isrotated--either directly with the actuating arm 113 (FIG. 1) or by anapply lever assembly 324 (FIGS. 7-10) which responds to the applicationof pressure applied by the operator of the brake pedal 12 of thevehicle. As shown, the apply lever assembly 324, if employed, may beoperatively secured to the apply shaft 323, as by a spline connection325. In either event, a spur gear 326 is provided on the inboard end ofthe apply shaft 323 meshingly to engage the teeth 328 presented on theradially inner surface of an annular apply cam member 330. The axialorientation of the teeth on the spur gear 326, as well as the axialorientation of the teeth 328 on the annular apply cam member 330 permitrelative axial movement therebetween, even while the teeth remain inmeshing engagement. The ability of the annular apply cam member 330 tobe readily displaced axially with respect to the apply shaft 323 whilethe two members remain in meshing engagement is quite important to theoperation of the brake apply assembly 300 utilizing a toothed inputmechanism, as will hereinafter become more fully apparent.

A plurality of ball bearings 331 are interposed between the race 332(FIGS. 8 and 11) presented from the annular apply cam member 330 and theopposed race 333 provided radially inwardly directed flange 334 on thebrake apply annulus 312. As will be hereinafter more fully explained,the ball bearings 331 will effect axial force transfer between theannular apply cam member 330 and the brake apply annulus 312, eventhough those two components are relatively rotatable. As will behereinafter described, axial translation of the annular apply cam member330, for any reason, will, through the application of axial force by theball bearings 331, tend to effect axial translation of the brake applyannulus 312.

As best seen in FIGS. 9 and 10, the radially outwardly directed surfaceof the apply cam member 330 is defined by radially offset, cylindricalfirst and second surfaces 335 and 336, respectively. A camming surface,indicated generally by the numeral 338, extends radially between thefirst offset surface 335 and the second offset surface 336. The cammingsurface 338 is comprised of a plurality of axially inclined apply ramps339--fifteen in the embodiment depicted--disposed between null peaks 340and return surfaces 341 such that each apply ramp 339 is inclined at anangle Φ with respect to a circumferential frame of reference 342, asbest seen in FIG. 11.

An annular reaction cam member 343 is disposed in axial opposition tothe apply cam member 330. The reaction cam member 343 presents anaxially disposed cam surface, identified generally by the numeral 344.The reaction cam surface 344 also comprises a plurality of axiallyinclined, reaction apply ramps 345--fifteen in the embodimentdepicted--disposed between null peaks 346 and return surfaces 348 suchthat each reaction apply ramp 345 is also inclined at an angle Φ withrespect to a circumferential frame of reference 349 which is disposed inparallel relation to the circumferential frame of reference 342 on theannular apply cam member 330, as best seen in FIG. 11. One apply roller350 is disposed between each of the opposed apply ramps 339 and 345 onthe apply cam member 330 and the reaction cam member 343, respectively,for a purpose more fully hereinafter described.

A containing skin 351 extends circumferentially about the reaction cammember 343. The reaction cam member 343, with the containing skirt 351positioned circumferentially thereabout, is received within the centralopening 352 through a response ring 353. A plurality of pins 354 extendradially through the response ring 353, the containing skirt 351 andinto the reaction cam member 343 in order to secure those members into asingle reaction assembly 355. A plurality of fastening means in thenature of machine bolts 356 (FIGS. 7 and 8) extend through the end wall358 of the brake housing 302 to be anchored in the response ring 353,thereby securing the reaction assembly 355 to the brake housing 302.

The response ring 353 presents a plurality of circumferentially spaced,axially outwardly extending lobes 359. As depicted, sixteen lobes 359would represent a typical embodiment. The opposed sides of eachsuccessive lobe 359 presents preferably planar self-energizing ramps 360and 361 that are each inclined at an angle θ with respect to an axialframe of reference 362, as represented in FIG. 11. An equal number ofvirtually identical lobes 363 extend axially outwardly atcircumferentially spaced locations about the outer rim 364 of the brakeapply annulus 312. The opposed sides of each successive lobe 363 alsopresents preferably planar self-energizing ramps 365 and 366 that arealso inclined at an angle θ with respect to an axial frame of reference368 thereon, as represented in FIG. 11, which is substantially parallelwith the axial frame of reference 362 on the response ring 353. Thereare preferably the same number of lobes 363 on the brake apply annulus312 as the number of lobes 359 on the response ring 353, although thelobes 359 on the response ring 353 are circumferentially displaced withrespect to the lobes 363 on the brake apply annulus 312. Thiscircumferential displacement positions the self-energizing ramp 360 oneach lobe 359 in opposition to the self-energizing ramp 365 on lobe 363.Similarly, the self-energizing ramp 361 on lobe 359 is thereby disposedin opposition to the self-energizing ramp 366 on lobe 363. A roller 370is disposed between each pair of opposed self-energizing ramps 359 and365 as well as each pair of opposed self-energizing ramps 361 and 366 onthe successive lobes 359 and 363.

As best seen in FIGS. 8, 9 and 10, an annular piston 371 extends axiallyoutwardly from the apply cam member 330 to be received within a mating,annular piston chamber 372 recessed within the end wall 358 of the brakehousing 302. A similar piston chamber is provided for that brake applyassembly 300 utilized with each transmission output shaft 301. Aspreviously described, pressurized hydraulic fluid is applied from thebrake apply valve 100 to the brake cylinders 101 through the feedconduit 144.

A plurality of displacement rods 373 are slidably received within acorresponding plurality of bores 374 circumferentially spaced about asupport flange 375 that extends radially inwardly from the brake housing302. The bores 374 may, as shown, alternate with mounting bores 376which also penetrate the support flange 375. The previously describedannular backing plate 321 may also be carried on the support flange 375.

The distal end 378 of each displacement rod 373 extends outwardly fromthe support flange 375 to engage the end face 379 on the skirt portion313 of the brake apply annulus 312. The opposite, or proximal, end 380of each displacement rod 373 is received within a chamber 381 for axialdisplacement. The chamber 381 contains means by which to provide abiasing protraction of the displacement rod 373 against the end face 379on the skirt portion 313 of the brake apply annulus 312. As shown, thatmeans may be the biasing action of a compression spring 382.

Operation of the brake apply assembly 300 is initiated when the vehicleoperator applies pressure to the brake pedal 12, which effects rotationof the apply shaft 323, either directly--or by virtue of a forcetransfer means, not shown, to the apply lever assembly 324. Rotation ofthe shaft 323, and the spur gear 326 secured thereto, rotates theannular apply cam member 330 and forces the apply ramps 339 to drive theapply rollers 350 against the reaction apply ramps 345 on the reactioncam member 343. Because the reaction cam member 343 is fixedly securedto the brake housing 302, the interaction between the apply ramps 339,the apply rollers 350 and the reaction apply ramps 345 translates theannular apply cam member 330 away from the reaction cam member 343 todrive the ball bearings 331 against radially inwardly directed flange334 on the brake apply annulus 312, thus also axially translating thebrake apply annulus 312 to compresses the associated brake pack 11between the flange 334 and the backing plate 321.

As the braking action between the interleaved torque plates 316 and 318begins to take effect, the torque applied to those torque plates 318rotating with the transmission output shaft 301 by virtue of thevehicular momentum is reflected back to the brake apply annulus 312. Thetorque is reflected through the interaction of the splines 315 on thefirst torque plates 316 with the splines 314 on the skirt portion 313 ofthe brake apply annulus 312. The resulting torque reaction is not,however, applied to the annular apply cam member 330 inasmuch as theball bearings 331 isolate the annular apply cam member 330 from rotationof the brake apply annulus 312. To the contrary, the torque feed back tothe brake apply annulus 312 does react against those rollers 370 locatedbetween the ramps 365 or 366 on the lobes 363 of the brake apply annulus312 which are, by the reflected torque, rotated toward the opposed ramps360 or 361 on the lobes 359 presented from the response ring 353. Theresulting interaction of the ramps on lobes 359, the rollers 370 and thereaction ramps on lobes 363 effects an additional translation of theannular apply cam member 330 away from the reaction cam member 343. Thistranslation also serves to drive the ball bearings 331 against theradially inwardly directed flange 334 on the brake apply annulus 312,thus effecting additional axial translation of the brake apply annulus312 to compresses the associated brake pack 11 even further. The brakeapplication resulting from the feed-back torque is designated as aself-energizing braking application, and its effect is additive to thebrake apply force initially generated by rotation of the annular applycam member 330.

With continued reference to FIG. 11, an in-depth understanding as to theoperation of the mechanical brake apply assembly 300 can be achieved byunderstanding certain mathematical relationships generated by thatassembly. The following mathematical terms are employed to express theforces present in the brake assembly:

F_(A) =The translation force applied by the brake apply annulus 312 inresponse to the interaction of the annular apply cam member 330 to thebrake apply annulus 312 through the apply rollers 350;

F_(SE) =The self-energizing force applied to the brake apply annulus 312in response to the interaction of the lobes 359 on the response ring 353with the lobes 363 on the brake apply annulus 312 through the rollers370;

F_(FB) =Return spring force in springs 382;

F_(C) =The total clamping force applied to the brake pack 11, which canbe mathematical expressed as:

    F.sub.C =F.sub.A +F.sub.SE -F.sub.FB                       (1)

In order to calculate the total mount of axial clamping force F_(c)required to generate a braking torque "T", one needs the followingadditional mathematical terms:

μ=The coefficient of friction between the torque plates 316 and 318 andthe interleaved friction disks 320;

R_(FP) =The mean radius of the friction disks 320; and,

N=The number of friction disks 320.

The axial clamping force F_(C) can then be calculated by themathematical expression: ##EQU1##

In order to calculate the self-energizing force F_(SE) generated by thattorque, one needs the following additional mathematical terms:

R_(SE) =The radius to the center of the self-energizing rollers 370;and,

θ=The angle of inclination of the self-energizing ramps 360 and 365 inone direction and 361 and 366 in the other direction.

The self-energizing force F_(SE) can then be calculated by the followingmathematical expression: ##EQU2##

Finally, one must determine the angle θ at which the system will notrelease--i.e.: the locking angle. The locking angle is theself-energizing ramp angle of inclination θ that results in aself-energizing force equal to the total force required to sustain thebraking torque. In order for the system to release, the braking forcemust release when the apply force is removed. Otherwise, the brakeswould lock every time the brakes were applied and would not releaseuntil the vehicle would be brought to a complete stop. Similarly, if thebrakes were applied when the vehicle was on a grade, the brakes couldnot be released without moving the vehicle up the grade. As such, thelocking angle θ is achieved whenever:

    F.sub.SE =F.sub.C                                          (4)

Expanding the foregoing mathematical expression, it will be observedthat: ##EQU3## Simplifying, ##EQU4##

The percentage of self-energization of a system is defined as thatpercentage of the total apply force that is provided by theself-energization feature. Typically, one would employ something in therange of about thirty percent (30%) self-energization. That is seventypercent (70%) of the damping force would be derived from the pressureapplied to the brake pedal 12 and thirty percent (30%) of the clampingforce would be self-generated from the system itself.

The higher the ratio of the self-energization braking force with respectto the mechanical apply force applied by the operator through the inputmechanism (such as the brake pedal 12), the more difficult the system isto control. That is, a large change in the total clamping forceresulting from a small change in the mechanical apply force normallyintroduces a degree of instability. For that reason, the percentage ofself-energization is kept well below fifty percent (50%), with aresulting apply force advantage of below 2:1. Such a ratio generatessufficient total clamping force, but it requires that a reasonablecomparable pedal force be applied by the vehicle operator.

The present system is designed to take advantage of theself-energization feature, and yet limit the amount of self-energizationto a desired range. This result is accomplished primarily by selectionof the inclination angle that is appropriately less than the lockingangle θ. For example, in a representative embodiment wherein the lockingangle θ is calculated to be 27.5 degrees, the actual angle selectedwould be on the order of about 13 degrees in order to utilize only aboutfifty percent (50%) of the self-energization feature. Even so, the brakepedal effort required to stop the vehicle under all conditions isgreatly reduced.

The apply system would be designed to impose an equal force to theannular apply cam member 330 associated with each axle, that force beingproportional to the pedal force and the linear stroke of the pedal 12.The displacement stroke of the pedal 12 adjusts the total force appliedto the brake packs 11 to attain a deceleration rate compatible with thetactile feed back to the vehicle operator through the brake pedal 12.The biasing pressure applied to the proximal end 380 of the displacementrods 373 acts against the mechanical force applied by theself-energization system to assure its release when the pressure appliedagainst the brake pedal 12 by the vehicle operator is release, orreduced.

The biasing force supplied by the springs 382 does not relieve the applysystem 300 of any of its load inasmuch as that biasing force is appliedto the isolated brake apply annulus 312 and not to the annular apply cammember 330. Hence, the biasing force relieves a portion of theself-energizing force at the rollers 370 but does not measurably affectthe position of the brake apply annulus 312.

It must be appreciated that if the hydraulic system were to fail, themechanical system would only be opposed by the biasing action of thesprings 382, and the vehicle could readily be brought to astop--although at perhaps a somewhat greater pedal pressure than wouldnormally be required. Finally, holding a vehicle on a grade with theengine off (no hydraulic system assist) would take advantage of theself-energization feature.

Modulator Signal Valve

With reference generally to FIG. 1 and, for more detail, to FIGS. 12through 17, the modulator signal valve 400 employs a spool member 401having lands 402 and 403 of different diameters. The first, or control,land 402 is of smaller diameter than the second, or valving, land 403and is axially slidable within a cylindrical, spool actuating bore 404that opens into an actuating chamber 405. A computerized control member13 communicates with the actuating chamber 405 by virtue of a controlpressure feed conduit 406 that opens to the actuating chamber 405through a port 408 that penetrates the cylindrical interior surface ofthe bore 404.

The computerized control member 13, as is well known, typically includesa regulator valve (not shown) that transmits a throttle responsivecontrol pressure through control pressure feed conduit 406 in responseto throttle position--the control pressure being proportional to thedegree to which the throttle is opened. In such arrangements the maximumcontrol pressure is supplied by the modulator valve in response to thedosed throttle--e.g.: a pressure on the order of 35 pounds per squareinch (0.2415 MPa)--and the minimum modulator valve control pressure issupplied in response to full throttle--e.g.: as little as zero poundsper square inch (zero MPa). This control pressure is fed to themodulator signal valve 400 through the control pressure feed conduit 406as the throttle responsive control pressure.

A first lug 410 extends axially outwardly from the spool member 401 toengage the opposed, end wall 411 of the actuating chamber 405 to assureunrestricted exposure of at least a portion of the first, or control,land 402 to the fluid pressure within the actuating chamber 405.

The second, or valving, land 403 is disposed in axially spaced relationto the first land 402, and as such, the second land 403 is axiallyslidable within a cylindrical, spool valving bore 412 that is coaxiallyaligned with the spool actuating bore 404. A directing chamber 413 isprovided between the first and second lands 402 and 403. Thelongitudinal spacing of the lands 402 and 403, and the axial translationof the spool member 401, are such that the directing chamber 413 remainsin continuous communication with a transfer conduit 414. As depicted,the transfer conduit 414 opens to the directing chamber 413 through atransfer port 415 that is located at the juncture of the spool valvingbore 412 with the spool actuating bore 404. A third branch 150_(C) ofthe main pressure supply conduit 150 communicates with the spool valvingbore 412 in spaced axial relation relative to the transfer port 415, toopen into the directing chamber 413 through an inlet port 418 when thespool member 401 is disposed as depicted in FIG. 12.

The spool valving bore 412 extends axially beyond the second, orvalving, land 403 to house a compression spring 419 that serves to biasthe first lug 410 toward engagement with the opposed end wall 411 of theactuating chamber 405, thereby tending to maintain the spool member 401in the position depicted in FIGS. 1 and 12 until the spool member 401has been otherwise actuated. With primary reference to FIG. 12, alocating pin 420 extends axially outwardly from the end cap 421 of thespool valving bore 412 in opposed registry with a second lug 422 thatextends axially outwardly from the modulator spool member 401.Engagement of the locating pin 420 with the second lug 422 defines themaximum extent to which the modulator spool member 401 can be axiallytranslated against the biasing action of the compression spring 419.

In order to assure that translation of the modulator spool member 401 isresisted solely by the action of the compression spring 419, the spoolvalving bore 412 communicates with the hydraulic return system 141through an exhaust conduit 423 that opens through port 424 in thecylindrical surface 416.

The directing chamber 413 also communicates with the hydraulic returnsystem 141. As depicted, a conduit 425 opens through exhaust port 426 inthe spool actuating bore 404 to communicate with the hydraulic returnsystem 141.

Cut-off/Timer Valve Assembly

The modulator signal valve 400 operatively communicates with a combinedcut-off/timer valve assembly 500. The valve assembly 500 has a cut-offvalve portion 501 that cooperatively interacts with a timer valveportion 502.

The cut-off valve portion 501 may employ a cut-off spool valve member503 having a pair of axially spaced, first and second lands 504 and 505which cooperatively engage the interior of a cylindrical valve bore 506to define a transfer chamber 508 between the first and second lands 504and 505, respectively. As depicted generally in FIG. 1, and with greaterspecificity in FIG. 12, the transfer conduit 414, which communicateswith the directing chamber 413 in the modulator signal valve 400, alsocommunicates with the transfer chamber 508 through an inlet port 509that penetrates the cylindrical valve bore 506. A throttle release, orpre-cooling, signal conduit 510 communicates with the transfer chamber508 through an outlet port 511. The outlet port 511 is located inproximity to the first land 504 when the cut-off spool valve member 503is disposed as depicted in FIGS. 1 and 12. A discharge port 512 opensinto the cylindrical bore 506 on the other side of the first land 504and serves to effect communication with the hydraulic return system 141through conduit 513.

The timer valve portion 502 has a piston 515 that is slidably receivedwithin a piston bore 516 that opens to an accumulator chamber 518. Apiston rod 519 extends axially outwardly from the piston 515 to beengageable with a head 520 that extends axially outwardly from the spoolmember 503 in the cut-off valve portion 501. A compression spring 521circumscribes the piston rod 519 and acts between the piston 515 and oneside 522 of a reaction shelf 523 on that portion of the housing 524which contains the cut-off/timer valve assembly 500. The spring 521continuously biases a spacer lug 525 on the piston 515 into engagementwith the opposed wall 526 of the accumulator chamber 518. By thusspacing the piston 515 from the opposed wall 526 unrestricted exposureof at least a portion of the piston 515 to the fluid within the chamber518 is assured, even when the compression spring 521 has biased thepiston 515 to its maximum disposition, as depicted in FIG. 1.

The head 520 on the cut-off spool valve member 503 is biased intoengagement with a shoulder 528 on the reaction shelf 523, as by acompression spring 529 received within the cylindrical valve bore 506.The spring 529 thus biasingly urges the cut-off spool member 503 tomaintain the transfer chamber 508 in communication with both thetransfer conduit 414 and the throttle release signal feed conduit 510.

A branch conduit 530 leads from the transfer conduit 414 to theaccumulator chamber 518. Pressurized signal fluid is fed through thebranch conduit 530 into the accumulator chamber 518 in order to actagainst the piston 515 and displace it against the biasing pressuresupplied by the compression spring 521. However, the pressurized fluidwithin the branch conduit 530 must pass through a restricting orifice532 in order to reach the accumulator chamber 518. The restrictingorifice 532 can be designed to serve as a time delay that is responsiveto the fluid pressure anticipated in the branch conduit 530 duringnormal operation of the vehicle. Typically, a one to two second periodof time will be desired between translation of the valving land 403 toopen port 418 in the modulator signal valve 400 and displacement of thepiston 515 in the cutoff/timer valve assembly 500 to close the transferchamber 508 to the admission of throttle release signal pressure to thesignal feed conduit 510.

Opening the port 418 admits throttle release signal pressure into thedirecting chamber 413 that will be transmitted to and through thetransfer chamber 508 and the feed conduit 510 to the piston chamber 210in the brake coolant valve 200. Subsequent displacement of the piston515 in response to fluid pressure within the accumulator chamber 518 ofsufficient magnitude to overcome the biasing action of spring 521effects closure of the transfer chamber 508 to the transfer conduit 414and opens the transfer chamber 508 to the hydraulic return system 141through the discharge port 512, and conduit 513.

In order to ensure that the cut-off spool valve member 503 and thepiston 515 are not undesirably restricted in their movement by theexistence of entrapped hydraulic fluid, conduit 533 effectscommunication between the hydraulic return system 141 and the valve bore506, and conduit 534 effects communication between the hydraulic returnsystem 141 and the piston bore 516.

Translation of the cut-off spool valve member 503 by piston 515 againstthe biasing action of spring 529 may be limited by providing a locatingpin 537 on an end cap 538 of the housing 524. The locating pin 537 isengaged by an opposed stop extension 539 on the valve member 503 todetermine the axial limit to which the spool valve member 503 can betranslated in response to translation of the piston 515, as depicted inFIG. 14. The stop extension 539 can also serve as a centering pedestalfor the spring 529.

It should also be noted that a unidirectional by-pass 535 is employed inthe branch conduit 530--in parallel with the restricting orifice 532.The by-pass 535 may, as shown, employ a spring biased check valvemechanism 536 that permits rapid depressurization of the accumulatorchamber 518 in response to a reduction of the pressure within thetransfer conduit 414, as will be hereinafter more fully described.

Finally, the throttle release signal feed conduit 510, whichcommunicates with the transfer chamber 508 through the outlet port 511,terminates in a ball-type shuttle valve 540 that is interposed at aconvenient location along the brake apply signal feed conduit 140 whichcommunicates between the brake signal chamber 110 in the brake applyvalve 100 and the piston chamber 210 in the brake coolant valve 200. Theshuttle valve 540 employs a ball 541 that is movable within a chamber542. The ball 541 is receivable in a first seat 543 that surrounds afirst port 544 through which the throttle release signal feed conduit510 enters the chamber 542, or the ball 541 is receivable in a secondseat 545 that surrounds a second port 546 through which the signal feedconduit 140_(A) opens to chamber 542. The second portion 140_(B) of thebrake apply signal feed conduit 140, which communicates with the pistonchamber 210 in the brake coolant valve 200, also communicates with thechamber 542 through a third port 548. The ball 541 is not designed toclose the third port 548.

Operation

As the vehicle is moving along, with the throttle applied (i.e.: thethrottle pedal is depressed to the position designated at 14 in FIG. 1),the throttle responsive control pressure supplied by the regulator valve(not shown) in the computerized control member 13 to the actuatingchamber 405 of the modulator signal control valve 400 is insufficient toeffect translation of the spool member 401. As such, the spring 419maintains the spool member 401 in the position depicted in FIG. 1.Accordingly, any residual pressurized fluid within the accumulatorchamber 518 will exhaust through the unidirectional by-pass 535, and thedirecting chamber 413, to enter the hydraulic return system 141 via theexhaust conduit 425 that opens to the directing chamber 413 through port426 in the cylindrical surface of the bore 404. Similarly, any residualpressurized fluid within the piston chamber 210 of the brake coolantvalve 200 will pass through portion 140_(B) of the brake apply signalfeed conduit 140, the shuttle valve 540, the signal feed conduit 510,the transfer chamber 508 in the cut-off valve portion 501 and thetransfer conduit 414 to enter the hydraulic return system 141, alsothrough the exhaust conduit 425 which leads from the directing chamber413.

Should the driver allow the throttle to close to that degree which wouldcause the pressurized fluid entering the actuating chamber 405 toovercome the biasing action of spring 419, as by lifting his foot fromthe throttle pedal such that it returns to the position represented at14', the modulator spool member 401 will translate against the biasingaction of the compression spring 419 such that the control land 402closes port 426 to preclude further access to the hydraulic returnsystem 141 through the directing chamber 413. Momentarily later, thevalving land 403 opens port 418 to admit main pressurized fluid into thedirecting chamber 413.

Thus, when the biasing force supplied by the compression spring 419 isovercome by the pressure applied against the control land 402 by thefluid within the actuating chamber 405, the modulator spool member 401will be moved to the position depicted in FIG. 13. With the modulatorspool member 401 so positioned pressurized fluid may enter the directionchamber 413 from branch conduit 150_(C) without restriction and continueto flow, as throttle release signal pressure, through the transferchamber 508 in the cut-off valve portion 501 and continue, unrestrictedthrough the signal feed conduit 510, the shuttle valve 540, portion140_(B) of the brake apply signal feed conduit 140 and into the pistonchamber 210 in the brake coolant valve 200, thereby permitting the brakecooling fluid to bath the brake packs 11 in preparation for theapplication of the brakes. The operation of the brake coolant valve 200was described in detail in conjunction with the description of the brakecoolant valve 200 herein, but the stippling in FIG. 13 depicts the flowof pressurized fluid through the system in response to the reduction offoot pressure on the throttle pedal 14.

With the system components disposed as shown in FIG. 13 one of twosubsequent operation will be effected. Either the brakes will be appliedor they will not be applied. To consider each alternative separately,let us first explore the operation of the combined cut-off/timer valveassembly 500 if the brakes are not applied.

Pressurized signal fluid within transfer conduit 414 will pass into thebranch conduit 530 and through the restricting orifice 532. The orificerestriction is designed to assure a predetermined period of time beforea sufficient volume of pressurized fluid can pass into the accumulatorchamber 518 of the timer valve portion 502 in order to effectdisplacement of the piston 515 against the biasing action of thecompression spring 521. That displacement of the piston translates thepiston rod 519 into engagement with the head 520 on the cut-off valvespool member 503. Continued translation of the piston rod 519 effectstranslation of the first land 504 on the cut-off valve spool member 503to the position depicted in FIG. 14, thereby terminating communicationbetween the transfer conduit 414 and the throttle release signal feedconduit 510.

As the first land 504 moves across the outlet port 511, the outlet port511 and the discharge port 512 will communicate across a portion of thevalve bore 506, thus allowing the signal feed conduit 510 to communicatewith the hydraulic return system 141 through discharge port 512. Aspreviously noted, the restricting orifice 532 can be designed inconjunction with the pressure of the fluid in the branch conduit 530,and with the strength of the compression spring 521, in order to effecta one to two second delay between the initial application of even amaximum modulator control pressure to the modulator signal valve 400 andthe closing of the cut-off valve portion 501. This is deemed to be asufficient time within which to maintain the application of an initialpre-cooling bath to the brake packs 11.

Should the vehicle operator re-apply the throttle, that action woulddecrease the throttle responsive control pressure fed from the controlmember 13 to the actuating chamber 405 of the modulator signal valve400. When that pressure is sufficiently decreased, the compressionspring 419 will exceed the differential pressure applied against thelands 402 and 403 by the mainline pressure of the fluid within thedirecting chamber 413. The spring 419 will then translate the modulatorspool member 401 such that the valving land 403 will close the port 418.Substantially simultaneously therewith the control land 402 will openthe port 426 to permit the pressure reflected within the directingchamber 413 from the pressure of the fluid within the transfer conduit414, and the conduit system still in communication with the transferconduit 414, to exhaust to the hydraulic return system 141 through port426.

Minor variations in throttle application have no affect. First, thestrength of the spring 419 may be selected so that a control pressure ofapproximately 20 pounds per square inch (0.207 MPa) will be required toovercome the action of the spring 419. Conversely, when the modulatorsignal control valve 400 has been actuated the differential areas of thelands 402 and 403 will come into play. Because the projected area of thevalving land 403 exposed to the directing chamber 413 is greater thanthe opposed, projected area of the control land 402, the pressure of thefluid within the directing chamber 413 will maintain the modulator poolmember 401 firmly against the locating pin 420 until there is asignificant drop in the pressure within the actuating chamber 405. Tocontinue the example, the differential projected areas of the lands 402and 403 in relation to the strength of the spring 419 is such that areduction in the control pressure to approximately 20 pounds per squareinch (0.207 MPa) will allow the modulator spool member 401 to bereturned to the position depicted in FIG. 1 by the biasing action ofspring 419. By requiring such a signal pressure to "reset" the modulatorsignal control valve 400, flutter, or cycling, is eliminated.

As soon as the signal pressure within the transfer conduit 414 is thusreduced, the timer valve portion 502 will reset. That is, thepressurized fluid within the accumulator chamber 518 will pass throughthe unidirectional by-pass 535 against the force applied by thecompression spring 521. The spring 529 is then free to translate thespool member 503 in the cut-off valve portion 501 to drive the head 520on the cut-off spool valve member 503 against the reaction shelf 523 inpreparation for the next throttle reduction. At this point thecomponents of the system 10 will all have returned to the positionsdepicted in FIG. 1.

On the other hand, should the components be disposed as depicted in FIG.13, and the vehicle operator would depress the brake pedal 12, the brakepacks 11 will have been provided with a pre-cooling bath by theoperation of the modulator regulating valve 400 operating in conjunctionwith the cut-off/timer valve assembly 500. Hence, as the brake pedal 12is depressed, say to the position 12' depicted in FIG. 15, the brakeapply valve 100 will be actuated by the initial displacement of thefirst spool member 102 to continue the flow of signal pressure frombranch 150_(A), through the brake signal chamber 110, the conduit140_(A) and into the chamber 542 of the shuttle valve 540. The signalpressure entering chamber 542 through port 546 will displace the ball541 from the second seat 545. Because the ball 541 is not designed toclose the third port 548, the pressurized signal fluid entering chamber542 from either branch 140_(A) of the brake apply signal feed conduit140 or the throttle release signal feed conduit 510 will flow throughthe branch 140_(B) to supply the piston chamber 210 in the brake coolantvalve 200 and thereby assure that the brake coolant valve 200 willcontinue to supply the desired cooling bath to the brake packs 11.

During the initial actuation of the brake apply valve 100 the modulatorsignal valve 400 and the combined cut-off/timer valve assembly 500 willhave "reset." Thus, the pressurized fluid in chamber 542 will attempt todischarge through the signal feed conduit 510--which then communicateswith the hydraulic return system 141--but the drop in pressure betweenthe chamber 542 and the signal feed conduit 510 will cause the ball 541to be received on the first seat 543 to seal the port 544 and therebypreclude further flow of fluid from the chamber 542 into the signal feedconduit 510, as shown in FIG. 15.

Continued depression of the brake pedal, as to the position depicted at12' will not only effect actuation of the brake apply valve 100 in themanner depicted in FIG. 4 to provide hydraulic actuation of the brakepacks 11, as heretofore described in conjunction with the description ofthe brake apply valve 100, but also rotate the apply shaft 323 toinitiate mechanical actuation of the brake packs 11, as previouslydescribed herein in conjunction with the explanation attendant uponFIGS. 4 and 7 through 11.

As should now be apparent, the present invention not only teaches that abrake system embodying the concepts of the present invention not onlyapplies a pre-cooling bath to the brake packs as a result of reducing,or releasing, the throttle but also that the other objects of theinvention can likewise be accomplished.

We claim:
 1. A brake apply control system comprising:a selectivelyoperable mechanical brake apply portion including, an operator linkagemeans including a rotatable input member for connecting the operatorwith a mechanical brake apply means, and a hydraulic piston and valvemeans including a linkage arrangement operatively connected with saidinput member for providing a hydraulic feedback force to the operator; aselectively operable hydraulic brake apply portion including, a sourceof hydraulic pressure, brake apply valve means for supplying a selectivebrake signal of variable intensity in response to an operator input,means connecting said source to said hydraulic piston and valve means,and means connecting said brake signal to said hydraulic piston andvalve means; and said hydraulic piston and valve means having anoperator-actuated piston means responsive to the brake apply valvesignal means and a sleeve valve in which is slidably disposed a reactionpin selectively connectible with the source of hydraulic pressure whenthe brake signal connecting means is operable to supply the brake signalto said hydraulic piston and valve means responding to saidsource-connecting means to provide a first feedback force to theoperator, and responding to said brake signal connecting means forproviding a second feedback force to the operator proportional to theintensity of the brake signal.
 2. A vehicular brake apply control systemfor providing first and second feedback forces to the operator of thevehicle in which the system is incorporated, said system comprising:aselectively operable mechanical brake apply portion; a selectivelyoperable hydraulic brake apply portion; an operator input linkage meansfor connecting the operator with said selectively operably mechanicalbrake apply portion and said selectively operable hydraulic brake applyportion; a source of pressurized hydraulic fluid; brake apply valvemeans for supplying a selective brake signal of variable intensity inresponse to an operator input; hydraulic valve and piston means forproviding hydraulic feedback to said operator input linkage meanscomprising:a housing; an operator input piston slidably received withinsaid housing; said operator input piston operatively connected to saidoperator input linkage means; a main pressure pin being axially drivenagainst said operator input piston by the fluid pressure from saidsource of pressurized hydraulic fluid to provide the first feedbackforce in response to operation of said mechanical brake apply portion;means connecting said source of pressurized hydraulic fluid to saidhydraulic brake apply portion; means connecting said source ofpressurized hydraulic fluid to said hydraulic piston and valve means;said hydraulic valve and piston means providing a first feedback forceto said operator input linkage means in response to operation of saidmechanical brake apply portion; and said hydraulic valve and pistonmeans providing a second feedback force to said operator input linkagemeans in response to operation of said hydraulic brake apply portion. 3.A vehicular brake apply control system, as set forth in claim 2, whereinsaid hydraulic valve and piston means further comprises:a stepped borewithin said housing; said stepped bore providing a first and a secondportion; said operator input piston slidable received within said firstportion of said stepped bore; a plunger sleeve valve member slidablyreceived within the other portion of said stepped bore; said mainpressure pin being received within said plunger sleeve valve member foraxial movement; passage means extending radially through said plungersleeve valve member to permit said source of pressurized hydraulic fluidto act against said main pressure pin in response to operation of saidmechanical brake apply portion.
 4. A vehicular brake apply controlsystem, as set forth in claim 3, wherein:said first portion of saidstepped bore is of relatively lesser diameter than said second portionof said stepped bore; said first and second portions joining at ashoulder; said operator input piston slidable received within said firstportion of said stepped bore; said brake apply valve means for supplyinga selective brake signal of variable intensity in response to anoperator input communicating with said second portion of said steppedbore in close proximity to said shoulder in order to drive said operatorinput valve means axially away from said plunger sleeve valve member inorder to provide said second feedback force to said operator inputlinkage means in response to operation of said hydraulic brake applyportion.
 5. A vehicular brake apply control system, as set forth inclaim 4, wherein said hydraulic valve and piston means furthercomprises:a spring means continually biasing said plunger sleeve valvemember toward said shoulder.
 6. A vehicular brake apply control systemfor providing first and second feedback forces to the operator of thevehicle in which the system is incorporated, said system comprising:aselectively operable mechanical brake apply portion; a selectivelyoperable hydraulic brake apply portion; an operator input linkage meansfor connecting the operator with said selectively operable mechanicalbrake apply portion and said selectively operable hydraulic brake applyportion; a source of pressurized hydraulic fluid; brake apply valvemeans for supplying a selectively brake signal of variable intensity inresponse to an operator input; hydraulic valve and piston means forproviding hydraulic feedback to said operator input linkage means; ahousing; a stepped bore within said housing; said stepped bore providinga first and a second portion; an operator input piston slidably receivedwithin said first portion of said stepped bore; a plunger sleeve valvemember slidably received within the other portion of said stepped bore;an axial bore extending through said plunger sleeve valve member; a mainpressure pin being received within said axial bore for axial movement; aspring means continually biasing said plunger sleeve valve member towardsaid shoulder; a radial passage penetrating said ;lunger sleeve valvemember and communicating with the axial passage extending therethrough;passage means penetrating said housing to effect communication betweenthat portion of said stepped bore receiving said plunger sleeve valvemember and said source of pressurized hydraulic fluid; said operatorinput piston engaging said plunger sleeve valve member and translatingsaid plunger sleeve valve member to effect communication between saidpassage means penetrating said housing, said radial passage in saidplunger sleeve valve member and said source of pressurized hydraulicfluid in conjunction with the operation of said mechanical brake applyportion and thereby effect the application of a first feedback forceapplied by said pressure pin to said operator input portion; said brakeapply valve means for supplying a selective brake signal of variableintensity in response to an operator input communicating with saidsecond portion of said stepped bore in close proximity to said shoulderin order to drive said operator input valve means axially away from saidplunger sleeve member in order to provide said second feedback force tosaid operator input linkage means in response to operation of saidhydraulic brake apply portion.